import java.util.Scanner;

public class EfficientPrimeNumbers {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.print("Find all prime numbers <= n, enter n: ");
    int n = input.nextInt();

    // A list to hold prime numbers
    java.util.List<Integer> list = 
      new java.util.ArrayList<>(); 

    final int NUMBER_PER_LINE = 10; // Display 10 per line
    int count = 0; // Count the number of prime numbers
    int number = 2; // A number to be tested for primeness
    int squareRoot = 1; // Check whether number <= squareRoot

    System.out.println("The prime numbers are \n");

    // Repeatedly find prime numbers
    while (number <= n) {
      // Assume the number is prime
      boolean isPrime = true; // Is the current number prime?

      if (squareRoot * squareRoot < number) squareRoot++;

      // Test whether number is prime
      for (int k = 0; k < list.size() 
                        && list.get(k) <= squareRoot; k++) {
        if (number % list.get(k) == 0) { // If true, not prime
          isPrime = false; // Set isPrime to false          
          break; // Exit the for loop
        }
      }

      // Print the prime number and increase the count
      if (isPrime) {
        count++; // Increase the count
        list.add(number); // Add a new prime to the list
        if (count % NUMBER_PER_LINE == 0) {
          // Print the number and advance to the new line
          System.out.println(number);
        }
        else
          System.out.print(number + " ");
      }

      // Check if the next number is prime
      number++;
    }

    System.out.println("\n" + count + 
      " prime(s) less than or equal to " + n);
  }
}